The conventional oscillators that equip mechanical timekeepers comprise, traditionally, a spring, or spiral, element making it possible to return a regulator element, or balance, to the neutral position. The power dissipated by the oscillation is offset by the application of a motor torque provided by a load spring, or barrel spring. However, this driving torque exerted by the barrel spring varies over time as a function of the load (or winding state) of the latter and, in most mechanical timekeepers, in particular when the barrel is coupled directly to the trains of the dynamic chain, this variation results in modifying the oscillation amplitude as well as, to a certain extent, the period of the oscillator. Such a modification can amount, for certain embodiments, to a deviation of one to several tens of seconds per day.
To offset the effect of the intensity variation of the motor torque, it was proposed to use a device called fusee (see the “Dictionnaire professionnel illustré de l'horlogerie” by G. A. Berner), which makes it possible to regularize the driving force transmitted to the train by the barrel spring. However, such a device is difficult to miniaturize, and therefore cannot actually be applied in mechanical bracelet watches.
Another correction device was described in relation to FIG. 7 of European patent application EP 1736838 in the applicant's name. In the latter document, it is proposed to have the motor torque of the barrel spring act on a flexible organ, which controls the active length of an element that participates in the continuous oscillation of the mechanical oscillator.
As in the case of the fusee, such a device is not simple to implement and, above all, neither of the two devices makes it possible to take torque variations into account that are due to friction existing, for example, at the different parts including the oscillator as well as the trains for transmitting the motor torque to the latter.
In quasi-continuous oscillation regime, i.e. when the intensity of the motor torque varies sufficiently slowly relative to the oscillation period, one can allow that the period variation caused is equivalent to that which would be caused by a non-linear return torque as a function of the deflection. Such an isochronism flaw can be corrected by an inverse non-linearity of the return spring.
The aim of the invention is thus to provide a corrector for the isochronism flaw caused by the variations of the motor torque of the barrel spring, according to a principle of correction as a function of the amplitude.
More generally, the aim of the invention is to be able to maintain a constant frequency of the oscillator, in its useful operating field, based on the amplitude variations to correct an effect that can be likened to a non-linearity of the return spring.